You might still be able to get external insulation - family member with '60s semi-D is getting it right now. They can paint, pebble-dash etc. and even have slip bricks if you have a partly brick facade (although I don’t know how good they are at matching brick colour - bit trickier than painting).
We looked into the external ourselves (detached house in a suburban estate) and it was going to be about 10k after the grant was deducted. We went for a cheaper alternative - no idea if it was a wise decision. I was put off by having to mess with drains and so on.
One gable wall and part of the front of our place has a cavity but the rest of the house doesn’t. Bit weird, but that’s how it is! So we pumped where we could and got internal insulation elsewhere.
Pros of pumped insulation, from our experience: it’s cheap and it doesn’t mess up the inside of the house. Downside: you don’t know for sure that the whole wall is filled with beads. They pump them in but who knows. We’re convinced that it’s not as good as the internal insulation - however, we’re mainly basing that on the living room which has an open fireplace, so maybe unfair on the beads…
The guys who did the beads also filled in part of the roof above the porch and above playroom. Left a hole in the ceiling of the playroom but we were getting a plasterer in for the internal insulation anyway.
Rooms with internal are great. It definitely works. We didn’t lose much space as the guy who did it pulled off the old, sub-standard layer of insulation and plaster and started again from the brick. It was very messy though and you obviously need to redecorate afterwards which is an expense and a pain. It seemed to take ages from start to finish - getting a painter to come was particularly painful, so get on that early.
We were doing this on an early '90s house we’d just moved into and that hadn’t been done up in 20 years so it felt like something we’d need to do anyway. If the decor was okay, it would have been off-putting to have to do so much at once. It wasn’t just plastering and painting, some of the built-in wardrobes had to come out to do the job right - and then we ended up having to replace them. They were past their sell-by date but that made for an expensive ‘insulation job’.
We also rolled out some insulation in the attic which seems to help. A neighbour did his roof with spray foam and likes it but I wonder whether you just trap lots of heat in the attic space. If the attic is a room, maybe that’s a good thing!
Hope that helps a bit. Just make sure you compare, as best you can, the likely total cost of each option - and your willingness to endure a big job if you go for internal insulation. Note that if you do a bit of X and a bit of Y that you can get the grant for whichever is the larger proportion of the house. In our case, and probably yours, you’d get the higher grant for the whole house - i.e. they treat it as though you’ve done internal insulation on the 100% house even though you only did 2/3 of the house.
Whatever you do will help a lot. Think we went from D2 to B something and it’s really good. Would need to tackle the open fireplace and a few other things to get into A territory.